It is a good book, although I don't agree with all of Bart's conclusions.  I think his description of the problems with modern media and the serious historian's problems with it are well said and easy to understand.  I think that chapter alone makes the book worth reading.

Sure.  We're going to come to some of the conclusions I disagree with Bart on here.

He contradicts himself a bit here.  If Mark, the foundation of Luke and Matthew's Gospels, was the originator of the story, how can he suggest that the similarities equate to "historical"?  Matthew and Luke copied these stories, just as Apollonius of Rhodes copied stories from Euripides and Homer; that does not validate a story.  It only shows that these traditions are accepted by the authors copying them.

How does one assume they are independent?  I feel Bart is overstating his case here.  I would imagine (in fact I know) that trying to establish independence is not easy to do.  Does Bart explain how he can claim something is established independently?  How much does his explanation or criteria revolve around speculation?

Which was probably Homeric, not "L".  Bart has not been keeping up with allusion criticism, I don't think.  Or perhaps he puts no stock in it.  In any event, not even Talbert in his discussion of Luke accepts the possibility of an "L" source.  We know which sources Mark used, and it did not come another "Q" or "L" or "M" source, like textual critics think.  It came from the Old Testament (or what books existed that have now come to make up the canonical Old Testament), Homer, Pauline theology, and that's pretty much it.  Everything can be traced back to these events, even if some scholars want to pretend they don't.  (I'm not saying that Bart is pretending, he may not have read anything about it)

A very good question.  I would imagine there isn't really an answer for that.  In my understanding of the evidence, Bart is wrong to assume this.

Richard Carrier (abstract pending) will most likely be tackling this theory in an upcoming essay.  However the short answer is, Bart and other historical critics do not consider the literary dependency on extant literature of the day, and instead have invented new literature which they assume existed.  There are whole seminars which deal specifically with Q which are nothing but hypothetical's and wishful thinking.

And one of the problems with a historical Jesus assumption is that deciding which things are made up and which aren't comes down to personal investments in the literature.  This is why there are rarely any scholars who have the same interpretation of who this "historical Jesus" was.  It is impossible to know, especially if you are already taking for granted historicity.  You now have to speculate and analyze things based on other speculations (birth place, parents, journeys). 

For example, did Jesus really go to Jerusalem ever, if he existed?  Well the Gospels say he did, but they resembled fictional stories created from the Old Testament, like his joyous entry on the donkey.  But if you assume his crucifixion under Pilate was real, he had to be in Jerusalem at some point.  So where is that incident?  When did he really enter and in what way?  Did he really stay in the garden which resembles more Old Testament trope use relating to the garden of Eden?  Well if not, how did he get betrayed, and where was he captured?  You see the problem?  When you assume something like this, more assumptions follow, and then whole arguments rest solely on assumptions and no evidence.  See my article on Jesus.

There is no way to know for certain.  The question is how much cumulative evidence supports the claims being made, and further, how many of those evidences are based on assumption, and formulated presupposing the argument?

Ehrman has made many mistakes in his critique of the Gospels and the evidence.  To answer your questions, (1) there is no evidence to verify independence and (2) my argument does not revolve around the possibility of independent accounts.

Regards

No problem.  Hope that helped.